In "wash filters", the liquid stream input is directed to flow along the internal face of the filter element. This tends to reduce deposition of retentions, which are swept to the far end of the filter, where they accumulate. Clogging of the perforations is thus delayed. Deflectors so placed as to accelerate the input flow of liquid along the surface of the filter element and thus increase the washing action, are well known. Because of the inevitable drop in the volume of the washing stream caused by outflow through the perforations, these deflectors are often bullet-shaped and designed so as to gradually narrow the annular stream and thus maintain velocity of flow.
The solid material driven to the end of the filter is not completely at rest. Local turbulence, sometimes surprisingly strong, tends to create a backflow, returning some solid material to the main stream, which in turn sweeps it back in a repeating cycle. This process may occur even when the accumulation is in a specially designed recovery zone or drifts into a sump attached to the housing. Consequently, even if the washing action along the internal face of the filter element is initially efficient in preventing or slowing blockage of the perforations, with time there is a gradual increase in the concentration of material moving freely and agitatedly in the liquid flow within the hollow of the filter element. This increases the chances that retained solids will be deposited on the surface of the filter element and possibly even forced into or through its perforations, leading to a reduction in output pressure and output flow and to the eventual need for cleaning or replacement of the filter element.
This problem may be obviated by special, energy-expensive wash filters, as proposed for certain oil and fuel filters (e.g. Verrando, U.S. Pat. No. 2,109,809, Meyer, U.S. Pat. No. 3,622,004), in which the volume of filtrate drawn off through the filter element is a small percentage only of the total flow into the filter, the majority of said flow being circulated through the filter housing but by-passing the filter element in a continual cycle incorporating a suitable sump. Such systems require (large and therefore heavy) and expensive filters and pumps, due to the high flow rates necessary.
More commonly, the build-up of retentions at the far end of most "wash filters" is flushed out by periodic operation of a manual or automatic valve. Some of the deposits on the wall of the filter element are expelled at the same time. These are loosened by the hydraulic shock and the increased flow within the filter element caused by valve opening. In many cases, however, this flushing does not prevent relatively rapid clogging of the filter, as some solid particles wholly or partially embedded in the filter element are not dislodged by this operation.
In U.S. Pat. No. 3,862,035, the current inventor disclosed a means for minimizing the accumulation of free or deposited solids in a filter, by continually removing from the filter the solids, without interrupting the flow of liquid through the filter. This prevents or minimizes hydraulic pressure loss and the costs of filter shutdown and cleaning, and for investment in backup. Generally, the latter invention comprised pressure-reducing means ("bleed") in communication with the filter body, the said bleed continuously discharging a small portion of the liquid flowing through the filter together with solids retained by the filter element, at a pressure substantially less than that within the filter. Various embodiments of the invention have greatly increased the working cycle of filters. They have also improved quality of filtration, by lessening eventual penetration through the filter element perforations of solids previously retained, though smaller than, or similar in size to the perforations, and of even larger solids, worn down by repeated particle-particle and particle-filter elements impacts, the latter caused by turbulence in the hollow of the filter element. Despite considerable improvements in performance and operational convenience provided by the latter invention for most liquid and particularly for wash filters, a number of problems remained.
Even when the load of solids in the liquid stream entering the filter element is relatively low and it is swept instantaneously to the filter end, turbulence may return some of the solids upstream as described, preventing immediate discharge through the bleed device. Furthermore, even when instant discharge of the bulk of retentions through the bleed does take place, there is a small increase in the number of free particles circulating indeterminately at the far end of the filter. Consequently, deposition and clogging usually start at the far end of the filter element in wash filters, even in those equipped with the bleed device. These processes are speeded when the input of solids to the filter is relatively heavy, and more so when solids enter in spurts. Increasing the liquid discharge rate through the bleed device is a partial solution, but this may be limited by a need for an economic and practical ratio between the total flow through the filter and the amount of liquid discharged through the bleed device.
In general, clogging of the filter element depends substantially on the concentration of retained solids moving around within the filter element at any given time. This in turn depends mainly on whether the discharge stream from the bleed is sufficient to remove solids swept to its vicinity. Even when there is a considerable, continual discharge through the pressure-reducing bleed, a free particle may not be evicted and thus remain for an extended period within the liquid contents of the filter element. However, it is useful to consider removal of liquid from within the filter element, as if the discharge were periodic and as if the volume evacuated per discharge was equal to the internal volume of the filter element (i.e. one Filter Element Evacuation, or one FEE). Thus, if the bleed flow is e.g. 500 liters/hour and the internal volume of the filter element is 2 liters, on the described theoretical and arbitrary basis, the liquid contents of said filter element together with retained solids, will be discharged 250 times/hour, or about four FEE/minute.
Traditionally, the internal volume of the kind of filter element referred to has been dictated by the required hydraulic and filtration characteristics of the filter element. It has not been considered as a parameter independent of the dimensions of the body defining it. Even when solid deflectors have been placed wholly or partly within it, these have been designed in relation to the filter screen, usually in order to increase or maintain the velocity of flow along the latter, or to achieve and maintain desired pressure characteristics through the filter screen. The extent to which they have reduced the volume of liquid within the filter element has been purely incidental.
It is one of the objects of the present invention to overcome the disadvantages of the prior-art filters and to provide a filter that is operatable for long periods without clogging.
This, the present invention achieves by providing a filter for separating solids from liquids, with means for the reduction of accumulation of solids within the filter and with automatic means for the continuous separation and removal at reduced pressure of liquid-entrained solids during filter operation, comprising:
a housing provided with an inlet for the liquid to be filtered, and provided with a first outlet for filtrate and with a second outlet for liquid containing a high concentration of solids;
a hollow filter element insertable into said housing, the internal side of said filter element being in fluid connection with said inlet and the external side of said filter element being in fluid connection with said first outlet;
a volume reduction device inserted in said filter element and being of a size to occupy a major portion of the hollow of said filter element;
a pressure reduction device in fluid connection with said second outlet, and provided with discharge means for solids and for the liquid in which they are entrained;
whereby said filter may operate for extended periods while clogging of said filter element is inhibited.
The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.
With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice .